Dead-burned magnesite and magnesia-alumina spinel refractory composition
BACKGROUND OF THE INVENTION 
     The present invention relates to refractory shapes and particularly bricks suitable for use in kiln operations utilizing lime rich clinker, more specifically, rotary lime and cement kilns. 
     Because of their excellent strength and resistance to spalling magnesite-chrome refractory shapes such as brick were extensively used in the rotary kiln industry. However, with knowledge that the chrome in such brick reacted with alkali in the kiln to form hexavalent chrome, a potentially hazardous material, the use of such type of brick has been greatly diminished. The reason for this is that as a potentially hazardous material it has to be disposed of accordingly which places a large additional cost on operators of such kilns. 
     Efforts to substitute other materials such as magnesite-spinel brick for this purpose have increased but such brick do not give a performance up to that o obtainable with the magnesite-chrome compositions. For example, the magnesite-spinel brick have a high thermal conductivity, as well as poor thermal spalling resistance particularly when used in vessels handling lime-rich clinkers. While some improvement in lowering thermal conductivity has been obtained, efforts to improve thermal spall resistance to a more satisfactory level has not occurred. 
     SUMMARY OF THE INVENTION 
     The present invention provides magnesite-spinel refractory shapes and particularly bricks with improved thermal spalling resistance, particularly in environments high with lime rich clinkers. 
     Briefly stated, the present invention comprises a refractory mix for forming shaped refractories comprising from about 60-85% by weight of a dead-burned magnesite and about 15-40% by weight of a magnesia-alumina spinel grain; said refractory having a total lime and silica content of about 2 to 4% by weight and a lime-to-silica ratio (CaO:SiO.sub. 2) greater than about 1. 
    
    
     DETAILED DESCRIPTION 
     The present invention is applicable to any refractory shape but will be discussed broadly with respect to the formation of brick, especially brick for use in rotary lime and cement kilns where lime-rich clinker is made. 
     The essential components of the instant refractory mix are the dead-burned magnesite and the magnesia-alumina spinel grain. It is the combination of these materials and the regulation of their lime and silica content and lime-to-silica ratio that enable the preparation of brick having a high thermal spalling resistance, particularly to lime rich clinkers. The lime-to-silica ratio should not exceed about 2.1 to 1. 
     As to the magnesite, it must be dead-burned magnesite, preferably one having a high lime-to-silica ratio; that is, a ratio above 1, with levels up to about 4 or 5 to 1 being suitable. This enables ready adjustment of the necessary lime and silica content and ratios in the final mix. As to the purity of the magnesite, it can have an MgO content of 85% and above but it is preferred to use higher purity magnesites; that is, those above about 95% MgO such as those having percentages up to 98% MgO. 
     As to the spinel, it can be a sintered or fused type spinel with the synthetic magnesium-aluminum spinel being preferred. As used herein, the term &#34;magnesia-alumina spinel&#34; is intended to distinguish from the other spinels such as the magnesia-chromia spinels which can be made synthetically. 
     It is of greatest importance to regulate the silica-to-lime ratio and to have a particular lime and silica content, as has been noted above, and such lime and silica particular ratios desired are preferably provided by the dead-burned magnesite which has the desired lime-to-silica ratio. In order to more readily prepare mixes with the proper ratio, it is preferred to make the fused magnesia-alumina spinel from magnesia and bauxite or high purity alumina. This eliminates the uncertainties with regard to the amounts of lime and silica in the spinel. 
     As to proportions, the burned refractory should contain from about 60-85% by weight of the dead-burned magnesite, preferably 75%, with the balance being the magnesia-alumina spinel grain. 
     The bricks can be formed simply by admixing the components, shaping the brick, and firing it using the equipment and processing conditions conventionally used for this purpose. 
     It will be understood that materials conventionally added to form the brick, including binders such as lignin liquors and release agents such as oil are utilized in the manufacture of the bricks of the present invention and are added in their usual amounts. 
     The invention will be further described in connection with the following examples which are set forth for purposes of illustration only. 
     EXAMPLE 1 
     A series of tests were carried out on three different mixes. The mixes were prepared by admixing the components and forming the brick therefrom using conventional brick-making apparatus. The brick were burned at 3150° F. 
     The components used and test results are set forth in Table I below. 
     
                       TABLE I                                                     
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PROPERTIES OF MAGNESITE-SPINEL                                            
BRICK BURNED AT 3150° F.                                           
Mix Designation:  A        B        C                                     
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Mix:                                                                      
Magnesite                                                                 
-4 + 10 mesh      17%      17%      17%                                   
-10 + 28 mesh     20       20       20                                    
BMF 70            34       34       34                                    
Fused Spinel (A-72)                                                       
-6 + 10 mesh      12       12       12                                    
-10 + 28 mesh     10       10       10                                    
-28 mesh          7        7        7                                     
Plus Additions:                                                           
Black Iron Oxide* --       3        5                                     
Lignin Liquor (SILICANIT)  3.5                                            
Oil                        0.5                                            
Density at the Press, pcf                                                 
                  191      192      192                                   
(Av 2):                                                                   
Degree of Sticking                                                        
                  Sl-      Sl-      Sl-                                   
on Burning*+:     Mod      Mod      Mod                                   
Linear Change on Burning, %:                                              
                  -0.4     -0.3     -0.4                                  
Bulk Density, pcf (Av 3):                                                 
                  191      191      191                                   
Data From Porosity (Av 3)                                                 
Bulk Density, pcf:                                                        
                  187      189      189                                   
Apparent Porosity, %:                                                     
                  15.9     15.9     16.0                                  
Apparent Specific Gravity:                                                
                  3.56     3.60     3.61                                  
Modulus of Rupture, psi (Av 3)                                            
At Room Temperature:                                                      
                  580      630      640                                   
At 2300° F.:                                                       
                  800      900      1020                                  
At 2700° F.:                                                       
                  230      170      170                                   
Load Test (25 psi) Subsidence                                             
After 90 Minutes                                                          
at 3100° F., %:                                                    
                  1.2      1.6      1.9                                   
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 *Added to determine if it had any effect on sintering and melt formation 
 in the brick.                                                            
 *+Mod--Moderate, Sev--Severe, Sl--Slight                                 
 
    
     The chemical analysis of the mixes used showed the following: 
     
         ______________________________________                                    
Mix Designation:                                                          
              A          B        C                                       
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 Chemical Analysis*                                                       
(Calcined Basis)                                                          
Silica (SiO.sub.2)                                                        
              0.60%      0.65%    0.67%                                   
Alumina (Al.sub.2 O.sub.3)                                                
              17.60      17.30    17.00                                   
Titania (TiO.sub.2)                                                       
              0.85       0.83     0.80                                    
Iron Oxide (Fe.sub.2 O.sub.3)                                             
              0.60       3.47     5.08                                    
Chromic Oxide (Cr.sub.2 O.sub.3)                                          
              0.19       0.18     0.18                                    
Lime (CaO)    1.22       1.18     1.14                                    
Boron Oxide (B.sub.2 O.sub.3)                                             
              0.030      0.030    0.032                                   
Total Analyzed                                                            
              21.09%     24.31%   24.90%                                  
By Difference                                                             
Magnesia (MgO)                                                            
              78.91      75.69    75.10                                   
Total         100.00%    100.00%  100.00%                                 
Lime to Silica Ratio:                                                     
              2.03       1.81     1.70                                    
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 *By xray spectrograph, except B.sub.2 O.sub.3 by emission spectrograph.  
 
    
     EXAMPLE 2 
     A trial run was made at a brick-making plant and the components used and results obtained on testing the brick are set forth in Table II below. 
     
                       TABLE II                                                    
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Works Trial Results on Mix A Burned at ˜2700°-2800°   
F.                                                                        
Mix Designation:    H-W 26-85                                             
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Mix:                                                                      
Magnesite (96A grade)                                                     
-4 + 10 mesh         22%                                                  
-10 + 28 mesh       15                                                    
-28 mesh            10                                                    
BMF 65              28                                                    
Fused Spinel -6 mesh                                                      
                    25                                                    
Plus Additions:                                                           
Lignin Liquor       3.3                                                   
(Lignosol B)                                                              
Lignin Liquor       0.5                                                   
(Lignosol BD)                                                             
Oil                 0.15                                                  
Density at the Press, pcf:                                                
                    187                                                   
Burn:               ˜2700°-2800° F.                   
                    Maximum-14 Cars                                       
                    Per Day                                               
Linear Change on Burning, %:                                              
                    -0.21                                                 
Volume Change on Burning, %:                                              
                    -1.60                                                 
Modulus of Rupture                                                        
At Room Temperature, psi:                                                 
                    540                                                   
Cold Crushing Strength:                                                   
                    3130                                                  
Chemical Analysis                                                         
(Calcined Basis)                                                          
Silica (SiO.sub.2)                                                        
Alumina (Al.sub.2 O.sub.3)                                                
                    14.5                                                  
Titania (TiO.sub.2) 0.5                                                   
Iron Oxide (Fe.sub.2 O.sub.3)                                             
                    1.1                                                   
Lime (CaO)          1.3                                                   
Total Analyzed      18.4%                                                 
By Difference                                                             
Magnesia (MgO)      81.6                                                  
Total               100.00%                                               
Lime to Silica Ratio:                                                     
                    1.3                                                   
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     The typical analysis of Magnesite 96A is: 
     
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Chemical Analysis:                                                        
(Calcined Basis)                                                          
Silica (SiO.sub.2)   0.7%                                                 
Lime (CaO)           2.2                                                  
Alumina (Al.sub.2 O.sub.3)                                                
                     0.1                                                  
Iron Oxide (Fe.sub.2 O.sub.3)                                             
                     0.2                                                  
Boron (B.sub.2 O.sub.3)                                                   
                     0.015                                                
Magnesia (MgO)       96.8                                                 
Lime-Silica Ratio    3.1 to 1.0                                           
Bulk Specific Gravity: gm/cc                                              
                     3.40                                                 
Screen Analysis:                                                          
Held on 1/2&#34;         53%                                                  
Pass 1/2&#34; on 4 Mesh  41                                                   
Pass 4 on 10 Mesh    3                                                    
Pass 10 Mesh         3                                                    
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     While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but, on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.